To answer your question, yes it is possible to duplicate your production environment in a lab setting as a method to plan a migration, but truthfully, why do you really want to waste your time doing this? Your going to open up a whole new can of worms trying to plan a migration in this mannor. You’ll run into hardware issues as Ghosting images from one hardware platform to another will generate configuration quirks, have you planned for addressing issues with DHCP scopes and DNS issues like your MX records? It’s not as simple as building a duplicate domain and plugging it in.

My suggestion to you is do some homework and plan a migration from your NT 4.0 domain to a Windows Server 2003 Active Directory domain. Microsoft has a multitude of helpful (free) tools to make the transition easy. Get your domain migrated and AD in place then plan your exchange migration. An W2k3 AD domain will support an Exchange 5.5 server in the interim so your not going to loose data or bring down your mail services while you plan. If your running one mail server, a simple method would be to take that extra server you had mentioned having and build that as your Exchange 2003 box and add it to your domain as a second Exchange server. Then do a mailbox move to migrate your mailboxes to the new server, decommision the old box, change your IP addresses to that of the old server and your pretty much good to go! There will be a bit more to it then that, but you get the gist…it’s not as hard as you think! Here is a link to a good step by step article to get you started…or do a Google search on migration and you’ll be surprised all of the info your going to find!